Saturday, November 19, 2016


Ceratosaurus (from Greek keras/keratos meaning “horn” and σαυρος/sauros meaning “lizard”), was a large predatory theropod dinosaur from the Late Jurassic Period (Kimmeridgian to Tithonian), found in the Morrison Formation of North America, and the Lourinhã Formation of Portugal (and possibly the Tendaguru Formation in Tanzania). It was characterized by large jaws with blade-like teeth, a large, blade-like horn on the snout and a pair of hornlets over the eyes. The forelimbs were powerfully built but very short. The bones of the sacrum were fused (synsacrum) and the pelvic bones were fused together and to this structure (i.e. similar to modern birds). A row of small osteoderms was present down the middle of the back.

Ceratosaurus, at first glance, looked like a fairly typical theropod, however its skull was quite large in proportion to the rest of its body, and large nasal and brow horns and possessed a prominent nose horn formed from protuberances of the nasal bones. In addition to the large nasal horn, Ceratosaurus possessed smaller hornlike ridges in front of each eye, similar to those of Allosaurus, these ridges were formed by enlargement of the lacrimal bones. Uniquely among theropods, Ceratosauruspossessed dermal armor, in the form of small osteoderms running down the middle of its back. Its tail comprised about half of the body’s total length and was thin and flexible with high vertebral spines.

The type specimen was an individual about 18 feet (5.5 m) long; it is not clear whether this animal was fully grown. David B. Norman (1985) estimated that the maximum length of Ceratosaurus was 20 ft (6.1 m), an assessment supported by a particularly large Ceratosaurus specimen from the Cleveland-Lloyd Dinosaur Quarry (UMNH 5728), discovered in the mid-1960s, which may have been 22 ft (6.7 m) long assuming similar proportions to the holotype.

Ceratosaurus is known from the Cleveland-Lloyd Dinosaur Quarry in central Utah and the Dry Mesa Quarry in Colorado. The type species, described by O. C. Marsh in 1884 and redescribed by Gilmore in 1920, is Ceratosaurus nasicornis. The first skeleton was excavated by rancher Marshall Parker Felch in 1883.

Artist's impression of C. nasicornis


Relatives of Ceratosaurus include GenyodectesElaphrosaurus, and the abelisaurs, such as Carnotaurus. The classification of Ceratosaurus and its immediate relatives has been under intense debate. Ceratosaurs are unique in their characters; they share some primitive traits with coelophysoids, but also share some derived traits with tetanuran theropods not found in coelophysians. Its closest relatives appear to be the abelisaurs.

In the past, Ceratosaurus, the abelisaurs, and the primitive coelophysoids were all grouped together and called Ceratosauria, defined as “theropods closer to Ceratosaurus than to Aves”. Recent evidence, however, has shown large distinctions between the later, larger and more advanced ceratosaurs and earlier forms like Coelophysis. While considered distant from birds among the theropods, Ceratosaurus and its kin were still very bird-like, and even had a more avian tarsus (ankle joint) than Allosaurus.